Grant Number 3R56AI069217-01A2W1 Kramer, Laura ARRA Funding Consideration Abstract Research is proposed to address PA-04-119 Biodefense and Emerging Infectious Disease Research Opportunities. WNV is a NIAID Category B priority pathogen. The introduction of a single WNV strain at a relatively well characterized time and place in North America has provided a unique opportunity to prospectively study the evolution of an arthropod-transmitted RNA virus as it adapts to a na[unreadable]ve ecological niche. While WNV in the Americas remains a relatively homogeneous virus population, a single genotype that differs from the introduced genotype has become dominant throughout North America, and since 2002 has displaced previously existing genotypes in the United States. Our studies suggest that the dominance of this new genotype is most likely facilitated by interactions between the virus and the mosquito host, but possibly also by interactions with natural avian hosts. The range of biological mechanisms that may lead to the displacement of one arboviral genotype with another, however, has not been systematically examined. Accordingly, we will test the hypothesis that dominance of one WNV genotype may result from selective advantages of this genotype in the mosquito. We propose experiments that will determine whether these phenotypic differences are associated with differences at the cellular level in viral replication events. We will determine the point(s) in the WNV replication cycle in critical mosquito vectors that are likely to have been responsible for the observed extinction of one viral genotype as it has been displaced with another in the U.S. Interactions of an arbovirus with both vertebrate and invertebrate hosts impact the intensity of virus transmission and thereby alter the rate of enzootic amplification, genetic stability of the virus, and subsequent viral fitness. Results from these studies will have direct implications on public health by increasing our understanding of the selective processes that shape invasive vector-borne virus populations using WNV as a model system for a vector-borne disease agent introduced into a naive environment. Such knowledge gains added importance in these times of rapid movement of people and commerce and concomitant movement of pathogens. A better understanding of the mechanisms of viral adaptation to naive environments and selection of genotypes in the host, will allow us to anticipate biological events and control introduced agents better.